TDA5250D2_07 Datasheet, PDF (44/94 Pages) Infineon Technologies AG – ASK/FSK 868MHz Wireless Transceiver

TDA5250 D2
Version 1.7
Application
3.1.4 Power-Amplifier
The power amplifier operates in a high efficient class C mode. This mode is characterized by a
pulsed operation of the power amplifier transistor at a current flow angle of θ<<π. A frequency
selective network at the amplifier output passes the fundamental frequency component of the pulse
spectrum of the collector current to the load. The load and its resonance transformation to the
collector of the power amplifier can be generalized by the equivalent circuit of Figure 3-6. The tank
circuit L//C//RL in parallel to the output impedance of the transistor should be in resonance at the
operating frequency of the transmitter.
V_S
LC
R_L
Equivalent_power_wmf.
Figure 3-6 Equivalent power amplifier tank circuit
The optimum load at the collector of the power amplifier for “critical” operation under idealized
conditions at resonance is:
R
=
V2
S
LC _2PO
[3 – 8]
A typical value of RLC for an RF output power of Po= 13mW is:
RLC = ³² = 350Ω
2 ∗ 0.013
[3 – 9]
Critical” operation is characterized by the RF peak voltage swing at the collector of the PA transistor
to just reach the supply voltage V_S. The high efficiency under “critical” operating conditions can be
explained by the low power loss at the transistor.
During the conducting phase of the transistor there is no or only a very small collector voltage
. ^{present, thus minimizing the power loss of the transistor (iC*uCE). This is particularly true for low}
current flow angles of θ<<π In practice the RF-saturation voltage of the PA transistor and other
parasitics will reduce the “critical” R_LC.
The output power Po will be reduced when operating in an “overcritical” mode at a R_L > R_LC. As
shown in Figure 3-7, however, power efficiency E (and bandwidth) will increase by some degree
when operating at higher RL. The collector efficiency E is defined as
Data Sheet
44
2007-02-26